Such a system will find application where it is desirable to make a recording of analogue, or other message, information such as conversations or other forms of audio or video that needs to be played back at a later stage, without the authenticity of the recording being called into question. Typical audio applications would occur with official law enforcement agencies, private investigation bureau, journalism, medical, legal and in many other fields. Application to video information would typically serve for verification of the authenticity of a video recording applicable in similar fields.
The storage of a conversation, message and other information, both analogue and digital, has previously been effected with a degree of verification to bear testimony of a past event or dialogue. U.S. Pat. No. 4,194,222 describes a voice logging communication recording system in which a digital time code is recorded together with the speech and a visual numerical display of the time code provided on playback. In this instance the time code is recorded out of phase between two channels of the recording medium to assist in cancelling the time code subsequently from the recorded audio. This system effectively bears testimony to the time at which the audio was recorded, but the recording medium could be tampered with in various ways to alter the audio and the system would be incapable of detecting such tampering.
Other systems are known which effect scrambling and enciphering of audio and data for recording, and subsequent unscrabmling and deciphering on playback. U.S. Pat. No. 4,004,089 discloses a system for recording a signal altered by a function on a magnetic medium, playing back the signal and operating on it with the same function to ensure accurate data. On the other hand, U.S. Pat. No. 4,193,061 is exemplary of systems used to determine the security of a transmission system. According to this specification, the device utilises a random process unit to generate identical functions at the transmit and receive ends of the system and these functions are then compared at the receive end of the system. U.S. Pat. No. 3,732,364 is of interest pirmarily with respect to its teaching of modulating a code signal with an information signal prior to recording and separating the signals during reproduction. Other forms of systems of this general kind are disclosed in the following U.S. Pats. Nos. 4,126,761; 4,042,565; 3,723,878 and 3,696,207. In many of the above disclosed systems, it will be noted that, a composite signal is produced comprising the audio, and recording thereof is effected on a signal track according to some systems and on multiple tracks of the recording medium in respect of other systems.
A system which has been developed to detect whether or not a recording has been the subject of any one of many forms of tampering is described in U.S. Pat. No. 4,477,848 having a common assignee with the present application. The specification of this application describes a system wherein a message signal is recorded together with a first data signal which is as least partly a function of the message signal, and upon playback a second data signal is produced which is at least partly a function of the message signal recovered from the recording. The two data signals are then compared and a comparison failure is indicative of an alatered recording. As described in that specification recording of the composite message and data signals is effected upon a single track of a magnetic recording tape. However, the compression into the confines of the band width of a single channel of a conventional recording tape has limited the quality of the message at playback and has detracted somewhat from the efficiency of operation of the system. Furthermore, although a high degree of resolution of tampering detection is provided by this system, improvement to embrace every conceivable form of tampering is desirable.